Efficiency and other performance criteria are driving higher the firing temperatures of combustion gas turbines in recent years. As these firing temperatures continue to rise, so is rising the requirement to improve the cooling efficiency of the blades, vanes, and other components subjected to the heat of the combustion gases in the gas turbine (collectively, “hot gas path components”).
Current firing temperatures easily are high enough to melt the metal alloys used for the hot gas path components. As a consequence of this, many such components are cooled using a gaseous cooling fluid passed through complex cooling channels within the component. The transfer of heat to the cooling medium, often compressed air or steam, cools the component. It is well known that some cooling is “open,” in that some or all of the cooling fluid is released through apertures into the component into the hot gas path, while other cooling is “closed,” meaning that no cooling fluid within the cooling channel system is so released.
Also, to further increase the efficiency of the cooling, a thermally insulating layer may be attached to the surfaces of the component exposed to the hot gas path or other sources of heat. The temperature gradient over this layer (one example of which is a Thermal Barrier Coating, or “TBC”) is high. This allows a reduction in the amount of cooling fluid needed in the cooling channels to attain a desired cooling effect and component temperature.
Since the strength of the metal alloy comprising a component declines as temperature rises, and since there is an efficiency cost in providing cooling fluid, it is beneficial to use the flow of cooling fluid as efficiently as possible. One approach to doing this is to provide flow paths in the cooling channels that are tortuous.
This approach, however, presents a challenge in the production of complex shaped, high performance hot gas path components having such tortuous and often complex cooling channels. Providing a tortuous flow path may include providing a pattern of irregular contours in the walls of the channels. For many cooling schemes that may include complex cooling channels comprising tortuous paths to increase cooling fluid efficiency, conventional single layer cores used in casting processes are not sufficient. That is, a single central core that defines the shape of a central cooling channel in a blade or other hot gas path component does not provide a basis for forming desired multiple and complex cooling channel designs.
Thus, one current fabrication approach to achieve a desired cooling channel complexity in hot gas path components is to form molds from a series of sliding blocks. These must be separated from each other to extract the core. Using this approach to produce complex three-dimensional shapes is difficult, and many desirable forms cannot be manufactured from single cores.
To use multiple layers of cores in conventional molding is time consuming and complex. The separate layers must be manufactured individually and then assembled precisely. Examples of current approaches to molding components include U.S. Pat. No. 5,250,136, issued Oct. 5, 1993 to K. F. O'Connor, and U.S. Pat. No. 6,901,661, issued Jun. 7, 2005 to B. Jonsson and L. Sundin.
In view of the above, there remains a need in the art for a method of producing a turbine component, particularly a hot gas path component, that comprises multiple layers of cooling channels wherein the production offers production cost savings while providing for complex cooling channel features and interconnects.